<DllImport("user32.dll")> _
Private Shared Function ShowWindow(ByVal hWnd As IntPtr, ByVal nCmdShow As Integer) As Boolean
End Function

Dim mywindow As Integer
Dim processRunning As Process() = Process.GetProcesses()
Dim Form As Object = New Object()
For Each pr As Process In processRunning
     If pr.ProcessName = "firefox" Then
          mywindow = pr.MainWindowHandle.ToInt32()
          ShowWindow(mywindow, 0)
     End If
Next

<DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
    Private Shared Function ShowWindow(ByVal hwnd As IntPtr, ByVal nCmdShow As Int32) As Boolean
    End Function

    Private Sub HideToolStripMenuItem_Click(sender As System.Object, e As System.EventArgs) Handles HideToolStripMenuItem.Click
        Const SW_HIDE As Integer = 0
        Dim p As Process() = Process.GetProcessesByName("firefox")
        If p.Length > 0 Then
            Dim hWnd As IntPtr = p(0).MainWindowHandle
            ShowWindow(hWnd, SW_HIDE)
        End If
    End Sub